An atomic force microscope (AFM) uses a probe in the form of a sharp needle that touches the surface of a sample and generates an image of that surface. In doing so, the probe picks up organic and inorganic debris from the surface and, as it picks up debris, the image resolution is decreased.
Similar types of probes are also used in scanning tunneling microscopes (STM). Like the probes used in AFM, the probes used in STM are often used very close to or touching the surface of a sample as the microscope scans the sample resulting in debris build-up on the probes.
In fact, the problem of debris build-up will occur with almost any type of probe that is used very close to or touching a surface and will also occur on many other types of surfaces, such as semiconductor wafers or pieces of glass.
One technique for solving the problem of a debris build-up on a probe is to simply replace the probe when image resolution has degraded below a tolerable level. Although this solves the debris build-up problem, it is an expensive solution.
A more practical and less expensive approach, is to remove debris build-up from a probe or other surface using a light source, such as ultra violet (UV) light in a UV light chamber. Although this approach works, the UV light UV light can damage light sensitive tip coatings and may cause the surface being cleaned to degrade.
Another approach is to prevent debris build-up from occurring in the first place by doing all of the imaging with the probe in a solvent. The solvent solubilizes any debris on the tip so that build-up does not occur. Unfortunately, for many types of experiments and applications a solvent can not be used because it may destroy the sample being examined.
Yet another technique for removing debris buildup specifically in STM probes is to generate an electric field in the STM probe itself so that the STM probe is somewhat "self-cleaning". Again this technique works, but each time it is used a portion of the probe is also removed. As a result, the probes wear out relatively quickly and must be replaced. Additionally, for this technique to work, the probe itself must be conductive, e.g., STM probes are conductive, but AFM probes are not.
Yet another technique is a procedure known as glow discharging where the surface to be cleaned is placed in a sealed chamber and a plasma is generated. The plasma is thought to neutralize the charges on the surface of the sample. Again this technique works, however it requires a chamber in which a plasma bath is generated and also requires the creation of a vacuum in the chamber or the introduction of ionizable gases in the chamber. The chamber is bulky and, thus not portable, and also is expensive.